Currently, in the security industry, dome camera assemblies are frequently used for video surveillance. The dome cameras may be used to monitor multiple areas in a place of business such as a bank, retail store, or casino. Typically, these surveillance systems may be monitored by an operator. The operator may select pre-programmed camera positions or repetitive sequences in rapid succession.
A dome camera assembly may include a housing, a camera, a bubble for the camera to look through, a pan motor, and a tilt motor. The camera may be a digital camera including a charge coupled device (CCD) for image capture. A microprocessor or digital signal processor (DSP) controls the pan and tilt motors by moving the position of the camera both horizontally and vertically. A typical dome camera assembly in the Closed Circuit Television Industry (CCTV) has the capability to pan the camera in one direction without ever having to stop and change direction. Furthermore, the camera typically has the ability to tilt at least 90° in the vertical direction. In many cases, the camera has a variable zoom lens, which satisfies applications where both a wide and a narrow Field of View (FOV) are required. Variability of the pan, tilt, and zoom positions of the camera give the operator the ability to view the entire hemisphere of the bubble.
A dome camera assembly usually has the ability to store pan, tilt, and zoom (PTZ) position values or prepositions into non-volatile memory. These stored prepositions or shots can be recalled at a later time either by an operator or automatically if an event is detected (e.g., an alarm triggered by a door opening). When a preposition is recalled, the microprocessor or DSP moves the camera to the pan, tilt, and zoom position that was stored in memory.
Today, the majority of dome cameras use stepper motors for pan, tilt, and zoom operations. Stepper motors have advantages over other motor types because the stepper motors can be controlled in an open loop system and because stepper motors do not require expensive sensing and feedback devices such as optical encoders. Stepper motors have predetermined step angles (e.g., 1.8°/step), and therefore, the position of the camera can always be known based on recorded input step pulses to the motors.
However, because such assemblies do not include a mechanism that provides position feedback, the actual position of a recalled PTZ preposition is assumed based on the step count. Any position errors are estimated based on design calculations and test measurements, but the actual position errors in the field are unknown. The position accuracy of a recalled PTZ preposition is known as repeatability.
The repeatability of a dome camera is based on a number of factors. The pan and tilt mechanisms often include reduction gears, belts, and linkage mechanism that couple the motor to the camera. Repeatability error of a recalled preposition may be due to hysteresis and backlash coming from the error buildup of the total pan/tilt drive mechanism.
Accordingly, a system is needed that is able to determine position error and adjust camera positioning in order to minimize repeatability error without increasing complexity and cost.